1. Field of the Invention
The present invention relates to a process and apparatus for controlling the melt level while pulling a single crystal from a melt according to the Czochralski process.
2. The Prior Art
When growing crystals according to the Czochralski process, a rod-shaped single crystal is grown from a seed. The single-crystal seed is brought into contact with a melt situated in a crucible and is slowly pulled away from the melt surface in an axial direction. In the process, molten material continually crystallizes at the interface between the single crystal and the melt and increases the volume of the single crystal. The pulling process is generally discontinued when the supply of molten material has been used up or a predefined crystal size has been reached. The success of crystal growing depends especially on regulating the temperature and the mass streams in the melt and in the gas space above the melt. The level of the melt surface with respect to an existing or an imaginary reference level has a decisive influence thereon, and the control of the melt level deserves particular attention.
Precise knowledge of the melt level is required, according to U.S. Pat. No. 4,915,775, so as to be able to set the crystal diameter as accurately as possible. It is therefore desirable to check the melt level during the entire pulling process, if possible, and to change it if necessary. Normally, the melt level, which would drop as liquid material is consumed, is kept constant by raising the crucible axially, in order to maintain the relative position of the heating elements, which are arranged to the side of the crucible, with respect to the melt surface. Recharging the crucible with solid or molten material can also be used to influence the melt level. By using this prior art process, a plurality of crystals can be pulled almost continuously. In the patent mentioned, the melt level is determined by measuring the time required to lower the seed crystal from a reference position until contact with the melt surface is achieved. A disadvantage of this method is that the melt level can be measured only once, prior to pulling the crystal, and cannot be measured continuously.
According to U.S. Pat. No. 4,454,096, the measurement of the melt level is achieved by directing a light beam onto the melt surface and having a sensor to register the light reflected by the surface. However, in this prior art method, the determination of the melt level of molten material in a reservoir crucible can be carried out only with great difficulty in a pulling crucible, because the single crystal obstructs the beam path.
The situation becomes even more problematic if a pulling process is practiced such as is described in U.S. Pat. No. 4,330,362. Here, there is a partial or virtually complete cover above the crucible from which the crystal is being pulled. Hence, it is virtually impossible to configure a light source irradiating the melt surface in combination with a sensor registering the reflected light.